M
edia F
ilter
Low Sediment Target Pollutants PollutantRemoval ReductionVolume Peak FlowReduction
Nutrients Organics
Trash Oil/Grease Metal
Bacteria
Moderate High Requires Pre-Treatment
chambers that are at ground-level and typically earthen. Because this filter type is flush with the ground plane, it is usually easier to maintain, but takes up more footprint area at a site.
Perimeter (subsurface) filters
, also known as Washington D.C. or Delaware sand filters, consist of two parallel trench chambers located in concrete vaults below an impervious surface, such as a parking lot. In this case, the pretreatment is built into the filter system as a sedimentation chamber that removes gross solids before storm- water passes to the filtration chamber.
The US EPA has more information about distinctions between types of sand filters. Commercially available sand filter products include the StormFilter and the VortFilter. Media filters containing both organic and mineral filtration materials generally have greater ion exchange capacity than sand filters, and therefore can more effectively remove soluble metals and other dissolved pollutants. This renders media filters particularly effective for roadways and highly industrial sites that contribute higher concentrations of metals to stormwater runoff, particularly zinc and copper. These filters have been shown to consistently remove over 85% of oil and grease, 82% of heavy metals, and around 40% of total phosphorus. There are also two main types of non-sand media filters:
Flatbed (surface) filters
wherein stormwater percolates vertically through a bed of media material exposed to open air at the ground surface.
Subsurface (drop-in) filters
wherein cartridges containing filter media are placed in concrete vaults located below grade, with the number of cartridges a function of design flow rates. Stormwater flows horizontally through the cartridge, then down- ward to an underdrain system. They are considered easier to maintain than flatbed filters.
Additionally, a range of proprietary media filters are also available, such as the Aqua- Filter Stormwater Filtration System, Contech’s StormFilter and BayFilter, and others. Both sand and other types of media filters are designed as either flow-based or volume- based treatment devices. Volume-based filters are designed with storage to completely capture the treatment volume and then filter it through the media within a desired drawdown time (typically 48 hours). Flow-based filters are designed with filtration rates that are sufficient to pass the treatment flowrate without requiring storage. Flow-based
designs require the use of specialized media with very high hydraulic conductivities, and therefore, flow-based designs are typically only used in proprietary media filters.
beneFItS
Effective removal of suspended solids, oil and grease, debris, and other attached
pollutants.
Effective removal of dissolved stormwater pollutants (media filters only); certain
types of media may target specific pollutant removal if desired.
Applicable in most soil types, appropriate in areas with poor infiltration.
Easily customizable to varying site size and dimension constraints.
Readily available materials (sand filters).
lImItatIonS
Minimal stormwater volume reduction, some peak flow attenuation.
Requires flat site and sufficient hydraulic gradient to support gravity flow.
Limited ability to remove dissolved pollutants such as soluble metals such as copper
and zinc (sand filters).
Provides low to moderate level of nitrogen removal and may increase nitrate levels.
SItInG
Drainage area and slope: The maximum drainage area treated by sand filters is typically 5 acres. Both sand and media filters are best suited for relatively flat sites (less than 5% slope), but must have sufficient hydraulic head (3 to 5 feet) to allow runoff to flow through the filter. The scale of the filter may affect its functionality; large scale systems like playing field filters may be more appropriate for detention and infiltration, while smaller filters can be more easily integrated into a dense urban environment as flow-through treatment facilities.
Depth to groundwater and bedrock: For sand and media filters that allow infiltration, the groundwater and bedrock should both be at least 4 feet below the base of the filter to prevent filter damage and groundwater contamination.
M
edia F
ilter
DeSIGn conSIDeratIonS
Materials: All types of media filters typically include 18 to 24 inches of filter media above a 12-inch gravel underdrain layer. In peat or compost systems, the filter media generally includes an upper layer high in organic content, an intermediate layer of peat mixed with sand, and a lower layer composed primarily of sand. Compost filters typically consist of an18-inch layer of mixed compost material. These organic media have a high cation exchange capacity which helps capture dissolved constituents. For inorganic media filters other than sand, most products are proprietary and will have manufacturer instructions to guide design considerations. Both the open and underground units require 3 to 5 feet of head to operate properly and are designed with overflows. Media filters can be lined with concrete or can infiltrate if the underlying soil has an infiltration rate of at least 0.5 inches per hour.
Pretreatment: As with other filtration systems, sediment will accumulate on the media surface, thus slowing the filtration capacity of the filter over time. To minimize sediment loading, the filter chamber should be preceded by a forebay or sedimentation chamber where suspended particles, oil, and grease can settle. Complementary BMPs such as vegetated swales, vegetated buffer strips, water quality inlets, and swirl separators can also help screen large particulates and trash prior to entry into the media filter.
Drainage: Media filters can connect directly to the collection system, can provide pre- treatment for detention or storage BMPs, or can infiltrate into the underlying soil matrix.
Overflow: Media filters can be designed as online or offline devices. In the online configuration, all flows from the contributing drainage area are routed to the media filter. Flows greater than the water quality event are captured by an overflow device and conveyed back to the collection system. In the offline configuration, only the volume treatable by the filter is diverted into the device; all excess flows are directed to the collection system. This configuration limits exposure to large storm events and erosive flows, which can shorten the effective life of the filter media. In areas with flooding or flow control requirements, it may be preferable to design media filters as offline devices and direct larger flows to detention or infiltration BMPs such as detention vaults or ponds and constructed wetlands.
Sizing: Media filters are sized using the volume-based calculations described in Step 7 of the Stormwater Control Plan chapter of the Guidelines. An electronic sizing tool is also provided with the Guidelines and is hosted on the SFPUC website at http://stormwater.sfwater. org in the Stormwater Design Guidelines section. The tool should be used for planning purposes only, such as determining the general footprint and preliminary dimensions. This calculator is not intended to provide final detailed calculations or dimensions. Volume- based sand filters are designed to store thetreatment volume within the sedimentation and filter chambers and to drain within a desired drawdown time, typically 24 to 48 hours. The available storage space includes the active sedimentation volume as well as the volume above the filter bed.
Some proprietary media filter systems are sized using flow-based calculations. The treatment capacity of the flow-based media filter should be greater than or equal to the treatment flowrate of the contributing drainage area. A simplified flow-based calculator has been provided on the SFPUC website (see above). However, it is highly recommended that the manufacturer be contacted directly and that their instructions be used to size these proprietary flow-based systems. As with volume-based filters, sedimentation pretreatment should be provided upstream of the filter.
operatIonS anD maIntenance
While sand filters are an ancient form of water treatment, other types of media filters are a relatively new technology, and precise maintenance procedures are still being refined. Typical maintenance of media filters involves replacing the top 2 to 3 inches of media every few years and cleaning out the sediment from the sedimentation chamber. This sediment is removed and disposed of in a manner similar to street catch basin maintenance. The table on the following page provides more information on typical post-construction inspection and maintenance activities.
M
edia F
ilter
Typical Inspection Activities for Media Filters
Activity Schedule
Check that the filter surface is not clogged and that the filter is
draining within the design drawdown time (typically 48 hours). Check that the storage chamber does not leak when standing
water is present.
Post-construction and semi-annually (beginning and end of rainy season)
Check to ensure that filter bed is clean of debris and that
sediment storage zone in sedimentation chamber is not more than 6 inches deep or 50% full.
Inspect grates, inlets, outlets, and overflow spillways for
clogging, erosion, cracking, or water damage.
Annually
Typical Maintenance Activities for Media Filters
Activity Schedule
Remove trash and debris from inlet, outlet, sedimentation
chamber, filter bed, and overflow devices to prevent clogging.
Semi-annually (beginning and end of rainy season)
Remove sediment from sedimentation chamber when depth
exceeds 6 inches or 50% of storage capacity. Annually or as needed Repair or replace damaged or clogged parts of filter fabric.
As needed
If water ponds at surface for more than 48 hours, clean or
replace top 2 to 3 inches of saturated / clogged filter media. Properly dispose of contaminated or saturated sediment after
cleaning - may require special disposal if contains metals, pathogens, or trace organic compounds.
As needed (expected to be >3 years)
coSt
Construction costs for a surface sand filter that treats a one acre drainage area are estimated at $18,500. Capital costs decrease with increasing drainage area. Perimeter sand filters tend to be less expensive, particularly if they are pre-cast rather than cast in place. Pre-cast perimeter media filters that treat a one acre drainage area range from $6,000 to $11,000. Annual maintenance costs average about 5% of the initial construction cost. The cost to replace the sand, filter fabric, and underlying gravel of a media filter is estimated at $1,700; however, a thorough and regular maintenance plan can significantly reduce these ongoing costs.
reFerenceS anD reSourceS
IDEQ. 2005. Compost Stormwater Filter. Storm Water Best Management Practices Catalog 39. September 2005.
Center for Watershed Protection. 2001. Stormwater Management Fact Sheet: Sand and Organic Filter. www.stormwatercenter.net.
California Stormwater Quality Association (CASQA). 2003. TC-40: Media Filters. California BMP Handbook- Municipal. January 2003.
Georgia Stormwater Management Manual, Volume 2 (Technical Handbook). Chapter 3.2.4: Sand Filters. Accessed 3 June 2008, < http://www.georgiastormwater. com/>.
United States Environmental Protection Agency. 1999. Wastewater Technology Fact Sheet: Sand Filters. EPA 832-F-00-023. September 2000.